1. Field of the Invention
The present invention relates to electrographic imaging and more particularly to improved apparatus and techniques for use in the development of electrographic images.
2. Brief Description of the Prior Art
At present the most common commercial technique for developing latent electrostatic images on dielectric supports is by contacting such images with triboelectrically-charged marking particles, in mixture with a particulate or liquid carrier medium. However, since the early stages in evolution of electrographic copying and printing techniques, it has been recognized that certain advantages pertain to development of latent electrographic images with small, airborne marking particles, e.g., in "liquid mist" or "powder cloud" form (hereinafter collectively referred to as "mists"). A primary advantage envisioned for mist development has been the achievement of higher resolution development and thus better quality in fine line detail and half-tone and continuous tone reproductions. A minimizing of wear on image elements was another advantage envisioned.
Early systems seeking to obtain such advantages introduced a mist of electrostatically charged marking particles between an image member (bearing an electrostatic image to be developed) and a development electrode that was spaced closely to the image bearing surface and connected to a source of potential which created an electrical field urging the charged particles toward the image. However, commercialization of such systems has been hampered by the tendency of precharged marking particles to mutually repel one another, causing great difficulty in production and transport of a uniformly charged and concentrated mist.
More recently electrographic imaging and development systems have been disclosed which utilize a mist of substantially-neutrally-charged marking particles, introduced between a uniformly-electrically-biased copy sheet and an apertured or grid array. Such arrays have been electrically-biased according to the image pattern to be reproduced and used to modulate the passage of an ion stream directed toward the receiver. The ions which pass through the modulator charge marking particles in their path, causing the particles to be attracted to and deposit on the receiver (see, e.g., U.S. Pat. No. 3,779,166). Although such neutral charge mist development systems obviate substantially the problems presented in use of precharged marking particles, they introduce certain other disadvantages and limitations. Specifically, the distance between the modulator and the receiver causes a loss of image sharpness. Also, the image resolution obtainable is limited by the opening size and spacings in the modulator, so that high resolution modulators are costly and highly susceptible to damage and clogging. Further, any continuous system of this type would require synchronization of the movement of the mist with respect to the receiver.
This invention is concerned with a new type of electrographic development which obtains the advantages of mist development yet avoids the disadvantages and limitations connected with prior art approaches.